In optical communication and the like, there are required variable optical attenuators, which are optical devices for controlling the quantity of transmitted light. Examples of variable optical attenuators include a reflection type variable optical attenuator as disclosed in Patent Literature 1 or the like. A reflection type variable optical attenuator is formed by optics arranged in a predetermined order along a direction in which incident light travels. Specifically, the optics are arranged as follows: A birefringent element 4, a convergent lens (convex lens) 5, and variable polarization rotation means 6, and a reflecting mirror 7 are arranged outside of an end of a dual-core ferrule 3 to which an input fiber 1 and an output fiber 2 are attached, in the order named from an incoming/outgoing surface of the dual-core ferrule 3. For the sake of convenience, the z-direction (rightward direction in the figure) is defined as a direction in which the optics are arranged (or incident light travels), and the x-direction (horizontal direction) and the y-direction (vertical direction) are defined as two directions perpendicular to the z-direction. Thus, FIG. 2(a) is a plan view, and FIG. 2(b) is a front view.
The input fiber 1 and the output fiber 2 are arranged in parallel to each other along the x-direction. In this example, the input fiber 1 is arranged on a right optical path as seen in the z-direction, and the output fiber 2 is arranged on a left optical path. The birefringent element 4 employs a plane-parallel birefringent element for separation and composition of polarized waves, which is capable of splitting, in the y-direction, light beams directed toward the z-direction on the same optical path with directions of polarization perpendicular to each other and of combining light beams directed toward the −z-direction on different optical paths. The reflecting mirror 7 is arranged at a focus point of the lens 5.
The variable polarization rotation means 6 includes a Faraday rotator 6a and an arrangement for applying a composite magnetic field of a fixed magnetic field and a variable magnetic field to the Faraday rotator 6a from two directions. The fixed magnetic field is applied in a direction in which the light travels by a disk-like permanent magnet 6b arranged behind the reflecting mirror 7. The variable magnetic field is applied in a direction perpendicular to the direction in which the light travels by an electromagnet 6c. Those two magnetic fields are applied to the Faraday rotator 6a, so that a Faraday rotation angle of the Faraday rotator 6a varies according to the composite magnetic field.
The principle of operation of the reflection type variable optical attenuator having the above configuration is as follows: Light emitted from the input fiber 1 passes through the birefringent element 4 and the lens 5, then converges on the reflecting mirror 7, and reflects from the reflecting mirror 7. The reflected return light passes through the lens 5 and the birefringent element 4 again and then exits. During this process, the light travels to and fro through the Faraday rotator 6a of the variable polarization rotation means 6. The quantity of reflected output light is controlled by adjusting a rotation angle of the direction of polarization with this variable polarization rotation means 6.
Specifically, light emitted from the input fiber 1 in the z-direction is optically split into an ordinary ray and an extraordinary ray in the y-direction by the birefringent element 4. Then the light is condensed by the lens 5. While the light is being condensed, it passes through the Faraday rotator 6a. When the Faraday rotation angle is zero degree, the light reflects from the reflecting mirror 7, which is located at the focus point of the lens, in a state in which the direction of polarization has not been rotated. Reflected light returning toward the −z-direction passes through the Faraday rotator 6a and the lens 5 again. At that time, the direction of polarization is not rotated. However, the positions of the ordinary ray and the extraordinary ray of the reflected light are shifted in diagonal directions from the focus point on the xy-plane. Then, at the birefringent element 4, all of the ordinary ray and the extraordinary ray are further split in the y-direction. Therefore, the incident light from the input fiber 1 is hardly coupled to the output fiber 2. In other words, almost the whole quantity of incident light from the input fiber 1 is attenuated.
Meanwhile, when the Faraday rotation angle is set to be 45 degrees, the light reflects from the reflecting mirror 7, which is located at the focus point of the lens, in a state in which the direction of polarization has been rotated through 45 degrees. At that time, the positions of the ordinary ray and the extraordinary ray are shifted in diagonal directions from the focus point on the xy-plane. The reflected light returning toward the −z-direction passes through the Faraday rotator 6a and the lens 5 again. At that time, the direction of polarization is further rotated through 45 degrees (thus 90 degrees in total). The direction of polarization of the light passing through the birefringent element 4 is rotated through 90 degrees. The ordinary ray and the extraordinary ray that have further been shifted in diagonal directions are combined in polarization with each other in the y-direction. Thus, almost the whole quantity of incident light from the input fiber 1 is emitted to the output fiber 2 while it is hardly attenuated.
Furthermore, the direction of polarization can be rotated through any angle with the variable polarization rotation means 6 through adjusting a magnetic field generated by the electromagnet 6c. For example, if the direction of polarization is adjusted so as to be rotated through 22.5 degrees, the light reflects from the reflecting mirror 7, which is located at the focus point of the lens, in a state in which the direction of polarization has been rotated through 22.5 degrees. The reflected light returning toward the −z-direction passes through the Faraday rotator 6a and the lens 12 again. At that time, the direction of polarization is further rotated through the same angle, or 22.5 degrees. Thus, the direction of polarization is rotated through 45 degrees in total. Then parts of the ordinary ray and the extraordinary ray are combined in polarization with each other in the y-direction by the birefringent element 4 and coupled to the output fiber 2. The rest of the ordinary ray and the extraordinary ray is further split in polarization in the y-direction and, therefore, is not coupled to the output fiber. Accordingly, if the Faraday rotation angle is set to be 22.5 degrees, the incident light from the input fiber 1 is attenuated so that the quantity of incident light is reduced almost by half. Then the light is emitted to the output fiber. In this manner, the amount of attenuation of the incident light (i.e., the quantity of reflected output light) can be adjusted as desired by controlling a rotation angle of the direction of polarization with the variable polarization rotation means 6.